1. Field of the Invention
The present invention relates to the inspection of welds using vibrothermography, and, more particularly, the present invention relates to inspecting and determining the quality of J-groove welds of a nuclear reactor pressure vessel (RPV) head and the RPV bottom mounted nozzles (BMNs) using vibrothermography.
2. Description of the Related Art
Nuclear reactors have a reactor cover or closure head through which a series of openings having tubes extending therethrough to the inside surface of the reactor are welded to the closure head by J-groove welds. These tubes have control rod drives and instrumentation packages sealably extending into the reactor internals. Similarly, the bottom portion of the RPV contains penetrations with instrumentation packages that are also connected to the RPV via J-groove welds.
Certain parts of the reactor vessel head are known to be susceptible to stress corrosion cracking. Many reactor heads have been completely replaced well before the planned design life because of extensive cracking in the nozzles and/or J-groove welds. The J-groove welds must be inspected during reactor shut down for any cracks that may have developed in the welds. Such inspections are usually made from beneath the reactor head with the head on a supporting head-stand during normal refueling periods. J-groove welds are difficult to inspect due to the complex geometry and high radiation fields under the head. Visual techniques are known to have difficulty with very small, tight surface cracks, especially on rough surfaces or after certain processes (i.e., grinding are used to improve surface finish for inspection. Standard nondestructive examination techniques such as eddy current and dye-penetrant examinations are the typical inspection approach used; however, application of these techniques is time-consuming in an operational environment where delays associated with inspections are undesirable. If ultrasound techniques are used for an RPV head inspection, the water couplant must be managed and disposed of following the inspection. Dye-penetrant techniques also suffer from disadvantages including requirements for cleaning the surface, a process that can involve significant volumes of solvent and chemicals.
High radiation under the reactor head makes it undesirable for personnel to perform these inspections by direct contact with the components; thus, various robotic elements are used to remotely access these welds with the mentioned nondestructive testing machinery. The robotic challenges with these techniques are significant, typically requiring a close proximity or actual contact to the tested weld surface which is difficult to achieve with the robotic delivery devices and the remotely controlled tools used to access the welds.
Thus, some form of nondestructive remote inspection of J-groove welds that did not require close proximity or contact with the welded surface, nor the exact robotic manipulation associated therewith (which is difficult to implement), was needed. One such form of testing is described in U.S. Pat. No. 6,856,662, the disclosure of which is incorporated herein by reference. This patent describes a photothermal inspection method, which includes sequentially heating small areas of a weld using a laser while monitoring the weld area using a remote infrared camera. The laser and camera are positioned at an angle of approximately 45° to the inspection surface normal. At cracks or other anomalies (referred to herein collectively as cracks), the thermal wave imparted by the laser does not propagate as rapidly, thereby producing an indication of the crack as a sharp drop in the thermal scan at the point of the crack. The temperature differential is detected using the infrared camera.
While the photothermal inspection method of the '662 patent is an improvement over prior J-groove weld inspection methods, it is desirable to use a nondestructive inspection system and method that allows for a quicker inspection.